1. Field of the Invention
This invention relates to a method of manufacturing a cylinder block of an engine.
2. Description of the Prior Art
There has been known a cylinder block which is cast from a light alloy such as aluminum. The cast cylinder block of the light alloy is less rigid than that of cast iron, and accordingly, when the journal portion of a crankshaft is directly supported by the light alloy, the bearing portion can be deformed and, at the same time, wear resistance and bearing strength are lacking. Thus it has been proposed, in Japanese Unexamined Patent Publication No. 61(1986)-45108, to cast bearing caps in a lower block and to fasten the lower block to a cylinder block body to support the crankshaft, thereby increasing the strength of the bearing portion.
Conventionally the cast-in type lower block is once fastened to the block body before the crankshaft and the like is incorporated, and the bearing portion is machined in arch a manner as to obtain a predetermined degree of roundness of the bearing portion. However, in such a conventional method of manufacturing the cylinder block, the roundness of the bearing portion can deteriorate when the lower block is removed from the block body, after machining, in order to incorporate the crankshaft and the like.
That is, the bearing cap cast in the lower block is stressed and deformed by the shrinkage stress produced as the aluminum alloy cools after casting. Even if the bearing portion is machined to a predetermined degree of roundness, with the lower block fastened to the block body to suppress the deformation under the stress, the bearing portion can be deformed and the roundness thereof can deteriorate under residual stress when the lower block is removed from the block body, since the strength of the bearing cap is reduced by the machining. Consequently, when the lower block is fastened to the block body with the crankshaft supported therebetween, the clearance between the bearing portion and the bearing metal cannot be uniform and an oil film cannot be uniformly provided.